The present invention relates to tile components that provide both thermal and impact protection for a reusable launch vehicle such as the space shuttle.
A reusable launch vehicle (RLV), such as the space shuttle, repeatedly travels into or beyond the earth""s upper atmosphere and then returns to the earth""s surface. During flight, the RLV experiences extreme temperatures, ranging from xe2x88x92250xc2x0 F. while in orbit to nearly 2500xc2x0 F. upon reentry to the atmosphere. Because of the extreme temperatures, the vehicle and its contents must be protected by a thermal protection system.
Thermal protection systems for RLVs are constructed from a large number, usually several thousand, of insulation tiles. The tiles function to insulate the vehicle from the environment and to radiate and reflect heat from the vehicle. In addition to protecting the vehicle from environmental heat sources, the insulation tiles also provide protection from localized heating sources such as the vehicle""s main engine, rocket boosters and directional thrusters. RLVs such as the space shuttle typically utilize a variety of tiles to cover the surface of the vehicle. Different areas of the vehicle encounter different heat profiles and different physical stresses during flight. Therefore, a variety of tiles having different compositions, densities, and coatings are placed at different positions of the vehicle depending on whether such positions are leeward or windward, upper or lower surfaces, etc.
The tile of a typical thermal protection system is shown in FIG. 1. The tile consists primarily of a thermal insulator 2, a relatively thick layer of material with an extremely low thermal conductivity. A typical thermal insulator material is Lockheed Insulation (LI), a light-weight fused silica fiber developed by Lockheed Missile and Space Company. LI is produced by mixing silica fibers having a diameter of 1 to 3 xcexcm and deionized water in a V-blender to form a slurry. The slurry is mixed with ammonia and stabilized colloidal silica solution after which it is placed in a casting tower where it is dewatered and slightly pressed to remove a portion of the water. The partially dried slurry is heated to a temperature of 250xc2x0 F. to remove the remaining residual water. The dried silica composition is then fired to a temperature of up to 2300xc2x0 F., which causes the colloidal silica to sinter the silica fibers to one another. The resulting insulative material is a low density mass of randomly arranged fused silica fibers. By selectively dewatering and pressing the silica fiber slurry, various densities of the resulting dry silica material may be produced. Lockheed Insulation tiles are marketed under the trade names LI-900(trademark), LI-1500(trademark) and LI-2200(trademark), having densities of 9 lb/ft3, 15 lb/ft3 and 22 lb/ft3, respectively.
Although relatively thick, the thermal insulator has a relatively low density and negligible resistance to Micrometeroids and Orbital Debris (NMOD). As a result, the thermal insulator provides insignificant protection in the event a particle collides with the RLV.
The exterior surface of the thermal insulator is covered with a thin, fragile outer coating 4, typically Reaction Cured Glass (RCG). RCG is produced by mixing fine ceramic powder in an alcohol solution and spraying this mixture onto the surface of the thermal insulator. After drying, the RCG-thermal insulator composite is heat cured at extreme temperature. The resulting product provides even greater thermal protection to the RLV than the thermal insulator alone. This outer coating is also very fragile and provides little protection from damage as a result of particles colliding with the RLV. Additional information on RCG may be found in U.S. Pat. No. 4,093,771 to Fletcher, et al, incorporated herein by reference.
The materials selected for both the thermal insulator and the outer coating are chosen for their extremely low thermal conductivity. As a result, they do not experience significant thermal expansion at elevated temperatures, nor do they experience significant thermal contraction at lower temperatures. In contrast, the RLV structure, typically aluminum, experiences significant expansion and contraction, up to thirty times that experienced by the thermal insulator, as a result of temperature variations. The RLV structure also undergoes relatively greater temporary distortion and deformation as a result of aerodynamic stresses than does the thermal insulator.
To accommodate for these differences in expansion and distortion, a Strain Isolator Pad (SIP) 6 is placed between the RLV 8 structure and the thermal insulator. The SIP is typically a flexible coated needled-felt Nomex(trademark) material. The Nomex(trademark) material provides excellent thermal characteristics and the needled-felt construction prevents tearing or damage during large lateral displacements. The SIP is bonded to both the RLV structure and the thermal insulator by any conventional method suitable for high temperature and high stress applications, such as the General Electric high temperature silicon adhesive RTV560.
Without the SIP, significant strain would develop at the interface between the thermal insulator and RLV structure, potentially causing damage to, or possibly loss of, the thermal insulation tile. Depending upon where the damaged or lost tile is located on the RLV, this could subject the RLV to an unacceptable risk of damage.
As stated previously, the primary purpose of the thermal protection system is to insulate the vehicle from the environment and to radiate and reflect heat away from the vehicle. Therefore, the components of the thermal protection systemxe2x80x94the outer coating, the thermal insulator and the SIPxe2x80x94are not particularly resistant to mechanical damage, particularly mechanical damage due to MMOD. Collisions of this type are a serious concern because objects of sufficient mass and/or velocity that collide with the RLV can potentially penetrate the RLV structure and destroy critical systems, which may lead to loss of the RLV.
In past years, there have been relatively few RLV flights and those flights did not last long. Therefore, the risk of damage due to an MMOD collision was small. As the frequency and duration of RLV missions continues to increase, however, the probability of MMOD collisions, and the need for protection from such collisions, becomes greater. Current estimates predict a 50 percent increase in shuttle flights, from 10 per year to 15 per year, between 2002 and 2012. Further, techniques presently used to avoid potential collisions, such as maneuvering the RLV out of the path of incoming particles or changing the orientation of the RLV to minimize the potential for damage, may not always be available in the future. For example, when the RLV is supporting the International Space Station, the RLV""s ability to maneuver and control its attitude will be limited. Therefore, it would be desirable to design a system that protects the RLV from both thermal and impact stresses. This system should preferably be lightweight to minimize the launch weight of the RLV.
In light of the foregoing background, the present invention provides an impact resistant insulation tile, and an associated method of protection, which protects an RLV from both extreme temperatures and the impact of MMOD. The impact resistant insulation tiles may completely cover the RLV or may be strategically placed in certain areas that pose a greater risk to the RLV if damaged.
The impact resistant insulation tile of the present invention is designed to reduce the kinetic energy of MMOD that collide with the RLV, thereby reducing the risk of serious damage to the RLV. This is accomplished by incorporating a layer or layers of material within the tiles that are capable of breaking apart objects that collide with the RLV. This process not only reduces the mass of these objects, it also reduces their velocity. As a result, the kinetic energy of the objects is significantly reduced.
The impact resistant insulation tile includes a durable coating applied to a surface of a thermal insulator. MMOD that collide with the impact resistant insulation tile are broken into smaller particles and their velocity reduced by the durable coating. According to one embodiment of this invention, a shock layer is embedded within the thermal insulator. MMOD that penetrate the durable outer coating with sufficient energy will continue to travel through the thermal insulator and impact the shock layer. The shock layer further fragments the MMOD particles, reducing the threat of damage to the RLV. Preferably, the velocity of these smaller particles is reduced sufficiently to prevent them from traveling completely through the thermal insulator and reaching the RLV structure. The fragmented particles that do reach the RLV structure pose a significantly lower risk of serious damage due to their reduced mass and velocity.
The impact resistant insulation tile also includes a SIP positioned between the thermal insulator and the RLV to accommodate the dissimilar thermal and mechanical characteristics of the thermal insulator and the RLV. In one embodiment of this invention, a Ballistic Strain Isolator Pad (BSIP) capable of capturing fragmented particles is used. As such, the SIP of this embodiment also serves as another line of defense against MMOD or the like.
Therefore, the tile of the present invention not only thermally insulates the RLV during flight, but also advantageously protects the RLV from MMOD. In this regard, MMOD that impact upon the tile are fragmented by the tile and captured by the SIP before impacting the RLV, thereby potentially increasing the life of the RLV and reducing maintenance costs. As a result of its construction, the tile is generally lightweight so as not to unnecessarily add to the weight of the RLV. Although the tile of the present invention is most commonly used on an RLV, the superior thermal and impact resistance properties of the invented tile could be utilized in any myriad of applications requiring a low density, highly insulative material.